To log on with the built-in Administrator account: Log off from your new account. Hit Ctrl + Alt + Del twice at the Welcome screen. Enter Administrator in the "Logon to Windows" dialog box.
For yourinformation, once you create a user account that is a computer administrator the built-in Administrator account will no longer be displayed on the Welcome screen. In order to restore access, you might want to take a look at this article:
How to login as Administrator in Windows XP http://windowsxp.mvps.org/admins.htm
You can also download the script on the right hand side of line 5 from this page to accomplish the same results:
Enable/Disable Administrator on Welcome Screen http://www.kellys-korner-xp.com/xp_tweaks.htm